Zinc oxide thin films have been used as a piezoelectric material for such devices as a surface acoustic wave device, a tuning fork vibrator or a tuning bar vibrator.
In FIG. 1, in side view, there is shown an example of a tuning fork vibrator having a zinc oxide thin film as a piezoelectric material. This tuning fork vibrator includes a main body 1 having leg portions 2 and 3, zinc oxide thin films 4 and 5 formed on the respective side walls 2a and 3a of leg portions 2 and 3 and Al base electrodes 6 and 7 formed on zinc oxide thin films 4 and 5. The zinc oxide thin films 4 and 5 can be formed by a chemical vapor deposition or a physical vapor deposition process such as vacuum deposition, sputtering or ion plating.
As the electrodes 6 and 7, Al is selected because it is inexpensive and can be easily bonded, and further, it is generally applied in I.C. The Al layer can be formed in a film thickness ranging between 3,000 .ANG. and 10,000 .ANG. by techniques such as electron beam heating deposition.
These conventional electrode structures for zinc oxide thin films, however, have the following defects. Since the Al base electrode itself has a high affinity, the Al diffuses into the zinc oxide thin film, causing a deterioration in the film's electrical characteristics. The diffusion of 3-valent Al in zinc oxide, a 2-valent semiconductor, causes the electrical characteristics of the zinc oxide thin film to deteriorate, e.g., it can seriously change its resonance frequency. When the Al electrode is subjected to a high temperature life test, the effect is accelerated and the deterioration of the electrical characteristics is further increased.
It has therefore been desirable to develop an improved Al electrode structure for zinc oxide thin films.